This invention relates to piezoelectric crystal resonators. More particularly, the invention relates to electrode patterns for piezoelectric crystal resonators operating in air.
Prior to this invention, there have been proposals for piezoelectric crystal resonators useful in various measurement tasks. They utilized a sample of piezoelectric material, having one or two vibrating beams. Thin coatings of conductive material were deposited in various configurations on the surfaces of the sample. The coatings formed an electrode pattern connected to an electrical oscillator circuit which excited the piezoelectric crystal to vibrate at its resonant frequency.
It also is known that the resonant frequency of the crystal changes when the crystal is subject to various stimuli. For example, if the crystal experiences an external force tending to deform the crystal, its resonant frequency changes and, consequently, the output frequency of an electrical oscillator connected to the crystal changes. This change in oscillator frequency can be sensed by conventional frequency discrimination circuitry to obtain an indication of the applied force.
Single beam quartz crystal flexure mode resonators are disclosed in U.S. Pat. No. 3,470,400 to Weisbord and U.S. Pat. No. 3,479,536 to Norris. Double beam flexure mode vibrators are disclosed in U.S. Pat. No. 3,238,789 to Erdley and U.S. Pat. No. 4,215,570 to EerNisse. All of the devices disclosed in these patents exhibit a high impedance at resonance. Moreover, the electrode patterns employed with the double beam devices do not effectively apply a lateral electric field along the entire length of the piezoelectric crystal beams. For example, the EerNisse patent discloses an electrode pattern which applies an electric field to only 44% of the beam length. These force transducers are used in a vacuum or sealed chamber filled with helium or other relatively inert gas. In some applications, however, it would be desirable to operate the resonator in air to reduce the cost of the instrument. It has been found, however, that the resonant frequency of these prior crystals is sensitive to humidity, which yields erroneous measurements in air.
Accordingly, a novel electrode pattern has been devised which lowers the crystal's impedance at resonance, effectively couples a lateral electric field to the crystal along its entire length and renders the resonance frequency relatively insensitive to changes in humidity.